1. Field of the Invention
This invention relates to a silicon pressure sensor chip, and more specifically to such a chip operating in a two wire resistive mode.
2. Description of the Prior Art
Silicon pressure sensor chips are well known in the art. Typical prior art sensor chips operate in the differential voltage mode. A piezo-resistive sensing element in the form of a Wheatstone bridge (four individual resistors) or a four terminal resistor (shear element), placed on a thin silicon diaphragm, yields a small output voltage of typically 20 mV/V full scale. This is the result of the piezo-resistive effect, which amounts to only a few percent of the resistance of the sensing element. A pressure sensor in the resistive mode requires a resistance change of up to 100 percent.
U.S. Pat. No. 4,079,351 issued Mar. 14, 1978 to Levine describes a pressure responsive sender whose electrical resistance varies in response to applied pressure. To achieve this function, a complex mechanical assembly of many parts is included. The pressure exerted by a medium in a pressure chamber deflects a diaphragm which moves a push member. This leads to a pivotal movement of a contact assembly around a knife edge and against the restoring force of a spring. The contacts engage the coil of a resistor assembly resulting in a decrease of the resistance with increasing pressure.
Several issues affecting manufacturing and reliability are addressed in the above disclosure. First, there is the problem of pressure hysteresis. Identical pressures will not produce an identical resistance after repeated pressure cycling due to the mechanical assembly with its bearings, friction and slip. Second, there is the problem of volume production and its associated cost when handling one device at a time. Also, there is the difficulty of precise calibration.
U.S. Pat. No. 4,543,457, issued Sep. 24, 1985 to Petersen et al. describes a microminiature pressure sensitive switch (shown in FIG. 1 in partial cross-section) based on silicon technology. A thin silicon diaphragm 100 is located opposite a metallized glass plate 102 and in response to an externally applied pressure diaphragm 100 bulges in the direction of glass plate 102 and makes contact with glass plate 102 at a well defined pressure. With increasing pressure, the contact area spreads out in the radial direction. At predetermined locations on the silicon diaphragm 100, there are button-like elevations 106 which define an electrical contact across the gap between silicon surface 100 and the surface of glass plate 102. These electrical contacts 106 are used as a switch for digitizing or monitoring a threshold level of an external pressure.
U.S Pat. No. 4,722,348, issued Feb. 2, 1988 to Ligtenberg et al., shows a silicon chip for a catheter tip pressure transducer have a rectangular diaphragm modified by the etching of two rectangular recess areas. This approximates a longitudinal beam, clamped at both ends, and loaded with a uniform pressure. The design has a small cross section, as necessary for a chip mounted inside a catheter with a small diameter.